To reduce the field-related interferences induced by radio systems, lightning discharges and other disturbing sources on the electrical and electronic systems in nacelles, an electromagnetic shielding is necessary. On one hand this is to ensure that the sensitive technology of the included switchgears is not disturbed, on the other hand, it serves to prevent personal injuries resulting from possibly occurring leakage currents or electrical discharges.
An important application field of such measures is nacelles of wind turbines, since sensitive technological systems are accommodated in a narrowest space which is accessible for maintenance personnel. Shielding is also necessary for implementing a lightning protection zone 1 within the nacelle according to the lightning protection zone concept described in the standards of the International Electro-technical Commission, IEC 62305 and required in the IEC 61400-24 for wind turbines.
For electromagnetic shielding in the nacelle of a wind turbine, various methods are known in the prior art: One possibility is to manufacture the nacelle completely from metal. However, this is undesirable in nacelles of large wind turbines due to the heavy weights and high material costs.
If the nacelle is made of composite material according to the prior art, a further possibility is to apply a metal foil on the inner or outer side of the nacelle cover or provide the nacelle cover with a lacquer coating with shielding characteristics.
A further possibility in most cases is to apply shielding mats on the inner side of a nacelle cover. An electrically conductive conductor layer configured as lattice mesh, which is integrated into the nacelle cover to provide shielding effect, is known from CA002657037A1.
Besides the electromagnetic shielding of the interior region of nacelles of wind turbines, it is prescribed according to the standard IEC 60364-5-54 that conductive structures, which are installed on the outside of nacelles, such as handrails, safety hooks, ladders or metal retaining systems for attachments, are equipped with means for potential equalization. These conductive structures must be connected to the potential equalization equipment of the machine support and the electrical equipment of the wind turbine. Because in this way it is prevented that a voltage equalization between electrically charged components and non-charged or components connected to the ground occurs via a person—for example, a maintenance person in the nacelle. The aim is to avoid dangerous contact voltages between two conductive parts in the hand area of 2.5 m.
This is usually realized by means of separate wires/cables, which are laid or mounted on the inside of the nacelle cover and more specifically on the walls and the roof. However, in addition to a growing complexity of the electrical equipment, this causes a potential risk, as a danger of stumbling or getting stuck occurs for the maintenance person in the case of improper fixation of wires and cables.
This complexity is additionally increased by the installation of cables of the lightning diversion device. The lightning current diversion occurs from the lightning rods on the roof of the nacelle cover of the wind turbine and is diverted to the earth by means of separately arranged diverting means, such as cables, wires and/or round wire. Mostly, these diverting means are led through the interior of the nacelle, which may have severe consequences for the systems in the nacelle in the case of the lightning diversion: Firstly, a lightning diversion through the interior of nacelles can result in an electrical flashover of the lightning diversion to an adjacent structural member, to electrical switchgears, cables and wires. Secondly, it can cause a radiation of an electromagnetic field in the case of diversion of high lightning currents, which induces currents and voltages in other conductor loops and conductive structures.
These currents (induced lightning current) and voltage (overvoltage) may endanger or destroy electrical equipment and electronic systems. The field components are not reduced in their field strength through the locally concentrated radiation of the field of the lightning conductor, and are carried almost unabated.
In the case of an isolated lightning current diversion and lack of connections for potential equalization, a punch-trough of the insulation may occur due to the high voltage drop on the wire. As a result in the isolation may burn up because of the high electric arc temperatures. This isolation fire may then spread to other systems and components, which can result in significant damage or destruction of the entire plant.
In cases of parallel guiding of lightning diverting devices or improper fixation of these conductors, electrodynamic transverse forces on the conductor also become noticeable. As a consequence, this could cause tearing of wires or damage to other systems and components through the moving conductor sections.
In the development, construction and installation of today's wind turbines, the electromagnetic shielding, the potential equalization and the lightning diversion are considered and executed separately.
An object of the present invention is to provide a nacelle cover for a wind turbine, which provides a solution for the aforementioned problems of the prior art.